Nanoclast iconNanoclast

New Method for Producing Molybdenum Disulfide Provides Two Potential Applications

Molybdenum disulfide (MoS2) started out as the next big thing after graphene in electronic applications. But that excitement started to wane somewhat when it was revealed that MoS2 contained traps—impurities or dislocations that can trap an electron or hole and hold it until a pair is completed—that limit its electronic properties.

While researchers continue to work on removing those traps in order to improve its electronic properties, others have been looking at uses for MoS2 outside of digital electronics in applications including photovoltaics and wearable electronics.

In this ever-expanding application universe, researchers at Rice University have found a method for manipulating MoS2 so that it could serve both as an improved catalyst for fuel cells and as the electrodes of supercapacitors.

In research published in the journal Advanced Materials, the Rice team, led by James Tour, developed a simple method for producing flexible films made from MoS2 that orients the material on its sides. In other words, have made the material in such a way so that the maxiumum amount of its edges are exposed.

The researchers showed that when oriented in this manner, the MoS2 can serve as an effective catalyst in the hydrogen evolution reaction (HER), a process used in fuel cells to pull hydrogen from water.

“So much of chemistry occurs at the edges of materials,” said Tour in a press release. “A two-dimensional material is like a sheet of paper: a large plane with very little edge. But our material is highly porous. What we see in the images are short, 5- to 6-nanometer planes and a lot of edge, as though the material had bore holes drilled all the way through.”

Tour added: “Its performance as a HER generator is as good as any molybdenum disulfide structure that has ever been seen, and it’s really easy to make.”

Other research has attempted to take advantage of MoS2 as a catalyst for fuel cells by standing them up on their sides. The Rice team took a different approach. First, they grew a porous molybdenum oxide film onto a molybdenum substrate through room-temperature anodization, an electrochemical process for thickening metal parts by adding a natural oxide layer.

The researchers then exposed the film to sulfur vapor at 300 °C (572 °F) for one hour. The result was molybdenum disulfide that had a flexible, nano-porous sponge-like structure.

Since the key to catalysts and to the electrodes in supercapacitors is surface area, the researchers immediately realized that the material would fit the bill for both applications. The Rice team developed a supercapacitor using the material and found the device retained 90 percent of its capacity after 10,000 charge-discharge cycles and 83 percent after 20,000 cycles.

Tour believes that this method of exploiting anodization could serve as a platform for a range of applications and devices.

“We see anodization as a route to materials for multiple platforms in the next generation of alternative energy devices,” Tour said. “These could be fuel cells, supercapacitors and batteries. And we’ve demonstrated two of those three are possible with this new material.”

Nanomaterial Promises to Reduce Shutdowns of Concentrating Solar Power Plants

Researchers at the University of California at San Diego have developed a composite nanomaterial that can convert 90 percent of the sunlight it captures into heat, making it an ideal candidate for solar absorption at concentrating solar power (CSP) plants.

"We wanted to create a material that absorbs sunlight [and] doesn't let any of it escape. We want the black hole of sunlight," said Sungho Jin, a professor at UC San Diego, in a press release.

The hybrid material, which is described in the journal Nano Energy,  combines copper oxide nanowires with cobalt oxide nanoparticles to create a multi-scale surface for the material with dimensions ranging from 10 nanometers to micrometers. This multi-scale surface gives the material its extraordinary efficiency at trapping and absorbing light.

Previous research has proposed the use of nanowires for concentrating the sun’s rays into a very small area of solar cells. And copper sulfide nanoparticles in combination with single-walled carbon nanotubes have proven effective in converting both light and thermal radiation into electricity.

In contrast to these other nanomaterials, which were intended for use in photovoltaics that convert light directly into electricity, the hybrid material developed by the UCSD team is being targeted for solar absorption to create heat. The heat is then used to boil water, which in turn creates steam that runs turbines to produce electricity.

The materials that are currently used for solar absorption lack the resistance to high temperatures that the job requires. As a result, they have to be replaced every year. This new hybrid material is unique in that it can withstand temperatures above 700 degrees Celsius.

This should make a significant difference for CSP plants that, as it stands now, have to shut down once a year to remove the degraded sunlight-absorbing material and apply a new coating. Because this means that there is no power generation occurring during this reapplication, the U.S. Department of Energy’s SunShot program was keen to support this most recent research to find a material that would have a significantly longer life span.

With CSP plants already producing approximately 3.5 gigawatts of power globally, the prospect of eliminating an annual shutdown and extending the maintenance interval to several years would make a big difference to the amount of electricity they produce and the confidence people have in this source of electricity.

Nanodiamonds Could Become Mankind's Best Friend

Researchers at Cardiff University in Wales have developed a new method for taking readings of processes going on inside living cells. The technique, which relies on nanodiamonds, could eventually aid in developing new modes of drug delivery and cancer therapeutics.

The traditional method for imaging cellular processes has depended on fluorophores, which are a fluorescent chemical compound that can re-emit light upon light excitation. But they degrade under the light used to illuminate them. This renders them ineffective as visualization targets after a short period of time. Furthermore, fluorophores have been shown to become toxic over time, sometimes killing nearby cells.

Nanodiamonds have been proposed as a one-to-one replacement for fluorophores for sometime now. But getting them to fluoresce required designing small defects into them. Manufacturing these defects into the diamonds proved to be costly and time consuming.

In research published in the journal Nature Nanotechnology, the Cardiff researchers developed a new method wherein the nanodiamonds are used differently, thus eliminating the need for this difficult trick of producing them with specific defects.

Instead, the Cardiff team demonstrated that nanodiamonds without defects can be imaged optically through the interaction between the illuminating light and the vibration of the chemical bonds inside the diamond lattice structure. These vibrations cause the light to scatter in such a way that they produce a different color.

To get this effect, the researchers used two laser beams pulsing at a specific frequency so that laser light triggers the chemical bonds inside the nanodiamonds to vibrate in sync. The researchers then focus one of two lasers at these vibrations, which produces a light known as, coherent anti-Stokes Raman scattering (CARS).

The researchers were then able to use a microscope to measure the intensity of the CARS light on a series of nanodiamonds of varying sizes. After using electron microscopy and other optical contrast methods developed by the researchers, the team was able to accurately measure the sizes of the diamonds. This then made it possible to quantify the relationship between the size of the nanodiamonds and the intensity of light that they produce.

The end result was a method that allowed the researchers to measure the size and number of nanodiamonds that had been delivered into the living cells.

“This new imaging modality opens the exciting prospect of following complex cellular trafficking pathways quantitatively with important applications in drug delivery,” said Paola Borri from the School of Biosciences, who led the study, in a press release. “The next step for us will be to push the technique to detect nanodiamonds of even smaller sizes than what we have shown so far and to demonstrate a specific application in drug delivery.”

DNA Can Carry Current, a Promising Step Toward Molecular Electronics

The promise of molecular electronics gets hoisted up the flagpole periodically, but now an international team of researchers based out of the Hebrew University of Jerusalem claim to have made a breakthrough with DNA molecules that they believe may be the most significant development in the last decade of molecular electronics research.

In research published in the journal Nature Nanotechnology,  a international group of researchers hailing from Cyprus, Denmark, Italy, Spain, and the United States has demonstrated that electric current can be transmitted through long DNA molecules. They believe that this demonstration could lead to the development of DNA-based electronic circuits.

Read More

Super-Black Nanotube Coating Could Reveal Space Obscured by the Sun's Glare

When astronomers want to see objects that are extremely faint, they call on powerful and sensitive instruments to make them clear. The problem is that these instruments are so sensitive, the slightest bit of stray light can simply overwhelm them, making it impossible to resolve the faint object over the other light.

To overcome this problem, telescopes and other deep-space imaging devices use something called a coronagraph, a telescope attachment that blocks out stray light coming into the telescope as the instrument attempts to resolve dimmer objects that may be washed out by the light.

This fall, NASA researchers on the International Space Station will test a new super-black carbon nanotube coating that promises to make these coronagraphs even more effective.

The NASA researchers were compelled to look for a new coating for coronagraphs to deal with the special challenges brought on by a new, compact coronagraph NASA had developed that is roughly half the mass, volume, and cost of today’s coronagraphs. But with this smaller size come greater demands on the instrument.

"Compact coronagraphs make greater demands on controlling stray light and diffraction," said Doug Rabin, a Goddard heliophysicist who studies diffraction and stray light in coronagraphs, in a press release. Rabin expects that the nanotube coating should be better at preventing stray light from reaching the focal plane of the instrument than the black paint currently used.

While space-based testing, meant to see how the coating responds to the environment of space is taking place, researchers at NASA’s Goddard Space Flight Center in Greenbelt, MD, will use the material to coat a cylindrically shaped coronagraph.

If both tests prove successful, the carbon nanotube coating would replace the black paint that is currently used on coronagraphs.  The enhanced coronagraph would not be limited to just the International Space Station, will likely also be used on commercial satellites that make critical space-weather-related measurements.

"We've made great progress on the coating," said Goddard optics engineer John Hagopian, in a press release. "The fact the coatings have survived the trip to the space station already has raised the maturity of the technology to a level that qualifies them for flight use. In many ways the external exposure of the samples on the space station subjects them to a much harsher environment than components will ever see inside of an instrument."

Skyrmions: Communication With Magnetic Swirls Instead of Electrons

A year ago, a team at the University of Hamburg showed that one could use skyrmions—tiny, swirling magnetic spin patterns in thin films—to store and erase data on magnetic media.

Because these patterns are much smaller than the magnetic domains in magnetic media such as today’s hard disks, this would raise the limit on data storage density. However, the team only proved that this could be done at a temperature of 4 K, and would require the application of a magnetic field to stabilize the patterns.

Now, Hamburg researchers have reported in Nature Nanotechnology how they created molecular magnets in a skyrmionic lattice without the need of an external magnetic field. These skyrmions, they say, also survive at room temperature. To form the skyrmion lattice, they used a single atomic layer of iron deposited on an iridium substrate. This layer has some interesting magnetic properties resembling antiferromagnetism. Looking at a small area of an antiferromagnetic lattice, you can observe two spins pointing up and two spins pointing down, and no net magnetic moment, explains Jens Brede, a physicist at the University of Hamburg and co-author of the paper.

“It is more complicated in a skyrmion lattice because the spins rotate, but roughly in a one-by-one-nanometer area you have as many spins pointing in one direction as the other, and therefore you have no net magnetic moment, and it does not react to an external magnetic field,” says Brede.

Now the researchers have found a way to create small magnetic islands on the skyrmion lattice by depositing single hydrocarbon molecules comprising six benzene rings atop the iron. “These molecules form a new way for the iron atoms directly below them to couple, and locally destroy the skyrmion, resulting in a tiny magnet in the skyrmion lattice,” says Brede.

These magnets, formally known as organic-ferromagnetic units, can have their magnetization switched up or down by an external magnetic field. “You can store information in them, but what is more interesting is that when you switch one of these little magnets, the skyrmion around it reacts,” says Brede. This has an interesting consequence: if you switch the magnetic field in one tiny magnet, the magnetization of another magnet at a distance of several nanometers will flip as well.

The skyrmion lattice is comparable to a compass array: a board carrying many magnetic needles that interact with each other like spins do. If you turn one needle, the other needles react by rotating to reestablish a lower energy level of the array. “This way you can transfer information from one magnetic molecule, through the skyrmion lattice, to the next one,” says Brede. “We saw this process of transferring information in this way for a distance of more than 10 nanometers; for magnetic interactions, this is a very long distance,” says Brede.

These magnetic interactions open the door for using the organic-ferromagnetic units in logic devices and information processing, says Brede. It is unlikely that these magnetic molecules could become qubits in quantum computers, but skyrmion lattices could still play a role in quantum computing.

“The system that we have now is absolutely flat. But we have a third dimension available. So we can imagine that instead of the particular molecule that we use, we use another molecule that contains a qubit. Then you have the skyrmion lattice to couple this qubit to another qubit in some part far away. You can imagine using that for parallel processing in quantum computing.” says Brede.

Long Live the Copper Qubit!

If a practical quantum computer is going to become an everyday thing, qubits have to remain in two states at one time for much longer than they do now. One of the possible candidates for a longer lasting qubit is a copper ion embedded in a large molecule.

A quick primer on qubits is in order. Certain ions have unpaired electrons whose spins can assume either of two spin states, up or down—or in computer speak, 0 or 1. But when struck with a microwave pulse, the unpaired electron can be coerced into assuming both the 0 and 1 state simultaneously. The two states are said to be in superposition. All the qubits created up to now stay in a superposition state for very short periods because the spin states of neighboring atoms quickly destroy the coherent state, making the life of the qubit too short for it to perform the desired number of quantum computations. But researchers have been looking high and low for suitable qubits and for ways to lengthen the periods over which they remain in superposition.

Now, a group at the University of Stuttgart reports, in the 20 October issue of Nature Communications, that it has developed a way to protect the spin of a copper ion by placing it inside a molecule that has relatively few spin-carrying atoms and keeping it far away from hydrogen atoms that carry spins. The copper ion is moved into a neighborhood where it’s surrounded by sulfur and carbon atoms that have no spin, and by nitrogen atoms that have a small magnetic moment. The team reports a coherence time of 68 microseconds at a temperature of 7 K—an order of magnitude better than what can be achieved with similar qubits now. The molecule can still function as a qubit at room temperature, although the coherence time is 1 microsecond. Qubits in nitrogen vacancies in diamonds have scored much longer coherence times, but “you cannot make a quantum computer with a single qubit,” says Joris van Slageren, the chemical physicist at the University of Stuttgart who led the research.

Read More

Nanoscale Structures Enable Magnetic Mirrors Without Metals

Researchers at Sandia National Laboratory have developed a two-dimensional array of non-metallic nanoscale structures that interact with the magnetic component of incoming light in such a way that they could enable a new generation of chemical sensors, solar cells, lasers, and other optoelectronic devices.

We are, of course, familiar with traditional mirrors, which interfere with incoming light so that they reflect back a reverse image that our eyes can see. However, what we don’t see is that these mirrors also interact with the incoming light so that its electrical field is reversed, essentially cancelling out the light’s electrical component.

But what if you didn’t want to cancel out that electrical component of the light and instead have nanoscale antennas at a mirror’s surface capture it so it could be used for a host of optoelectronic devices? The way to do that has been something called “magnetic mirrors,” which interact with light’s magnetic field and preserve light’s original electrical properties.

Since nature does not provide a material for producing magnetic mirrors, researchers have turned to metamaterials, which can be broadly defined as an artificially structured material fabricated by assembling different objects so as to replace the atoms and molecules that one would see in a conventional material. The resulting material has very different electromagnetic properties than those that occur naturally or are chemically synthesized.

Prior to this most recent research, metamaterials could only produce this magnetic mirror effect for microwave frequencies; this limited it to a small number of applications such as microwave antennas.

At wavelengths shorter than those at microwave frequencies, there has been some success in using “fish-scale” shaped metallic components. However, this approach has suffered from loss of signal and uneven responses.

In the research, which was published in the journal Optica, the Sandia team used tellurium to reduce the signal loss that is associated with metals. The telluride-based nanostructures also resulted in magnetic mirrors that were reflective at infrared wavelengths and led to a much stronger electrical field at the mirror’s surface.

“We have achieved a new milestone in magnetic mirror technology by experimentally demonstrating this remarkable behavior of light at infrared wavelengths,” said Michael Sinclair, a Sandia scientist who co-authored the Optica paper, in a press release.

“Our breakthrough comes from using a specially engineered, non-metallic surface studded with nanoscale resonators.”

What may make the approach most attractive is the fact that it can be produced with widely used deposition lithography and etching processes.

In future research, the team intends to investigate other materials to reveal their magnetic mirror behavior with an eye towards getting to shorter optical wavelengths. This should expand the optoelectronic applications.

“If efficient magnetic mirrors could be scaled to even shorter wavelengths, then they could enable smaller photodetectors, solar cells, and possibly lasers,” said Sheng Liu, Sandia postdoctoral associate and lead author on the Optica paper, in the release.

Platinum Catalysts Are Outshined By Graphene Quantum Dots

Platinum is widely used as a catalyst for oxygen reduction reactions in fuel cells, but its high cost is a major obstacle to making fuel cell vehicles more affordable and more popular. Graphene, however, may be just what automotive and energy companies are looking for

Researchers from Rice University attached graphene quantum dots to a graphene base, resulting in a hybrid material that operates as an excellent– and cheap–catalyst for fuel cell reactions.

Quantum dots are nanocrystals that exhibit quantum mechanical properties, making them very useful in experimental transistors, solar cells, imaging chips, and other things. James Tour, a Rice chemistry professor, and his colleagues created graphene quantum dots (GQDs) from coal last year, and followed up on that breakthrough with this latest experiment.

This time, those same GQDs were combined with microscopic sheets of graphene into self-assembling nanoscale platelets. Tour explains that in the hybrid GQD/graphene material, the quantum dots provide a high abundance of edges where chemical reactions can occur, while the graphene is a plane of conductivity between GQDs. (The material was also treated with boron and nitrogen–co-catalysts.)

The lab discovered that the new material actually outperforms platinum-based catalysts for fuel cell reactions. Compared to platinum, the GQD/graphene material showed an oxygen reduction reaction with about 15 millivolts more in positive onset potential, or the start of the reaction. “You don’t need to apply as high a voltage as platinum to get the oxygen reduction reaction to occur,” says Tour. “We also get about 70 percent higher current than what platinum would offer.”

Increased efficiency aside, the hybrid material is also cheaper to make and install in fuel cells. The average cost of a fuel cell for an automobile has gone down from $275 per kW capability in 2002, to $55 per kW in 2013. Yet the fuel cell industry has yet to make a profit. The U.S. Department of Energy believes automotive fuel cell costs will need to fall to $30 per kW before we can expect to see real consumer interest. Toyota began selling its hybrid hydrogen fuel cell car in Japan this year for around $68,600. Unfortunately, with a 2014 Corolla going for between $16,000 and $21,000, fuel cell vehicles have yet to be noticed by most consumers.

The researchers believe the new findings, published in ACS Nano, could pave the way toward that goal. “This allows one of those two key reactions in the fuel cell to happen very well for the formation of water,” says Tour. “With fuel cells, the concern is always the cost of platinum electrode. This obviously needs no platinum electrode.”

However, Tour is very hesitant to say the fuel cell era is just over the horizon, and with good reason. “We don’t have an infrastructure as a country or global system for exploiting hydrogen-based systems well. You have to have a buy-in from everyone involved in that system for things to start taking off.”

Bottom-Up Self Assembly of Graphene Holds Promise for Spintronics

Not all graphene is alike. The way in which graphene is produced determines in large measure how it can be applied. The aim, of course, has been to produce the best quality graphene in large quantities.

However, these bulk production methods come at a price, which usually involves compromising those astounding electronic properties that make graphene so attractive in the first place.

Now researchers at the University of California Los Angeles (UCLA) and Tohoku University in Japan may have found a way around these limitations by abandoning “top-down” manufacturing techniques like lithography for a bottom-up approach in which the graphene nanoribbons self assemble exactly into the desired form.

The researchers were looking for a way to produce graphene nanoribbons that have the zigzag edges that give the material a strong magnetic property, making it attractive for spintronics. Spintronics exploits the way in which the spin of particles respond to magnetic fields so that the spin is either parallel or antiparallel to the magnetic field. These two possibilities make it useful for creating a digital signal that can be used in computing.

“To make devices out of graphene, we need to control its geometric and electronic structures,” said Paul Weiss of UCLA in a press release. “Making zigzag edges does both of these simultaneously, as there are some special properties of graphene nanoribbons with zigzag edges. Having these in hand will enable us to test theoretical predictions about them, such as magnetic properties.”

The typical lithographic method for producing graphene nanoribbons with these zigzag edges resulted in too many defects in the final product for the material to be useful.

In the research, which was published in the journal ACS Nano, the team exploited the properties of a copper substrate to alter the way the graphene precursor molecules reacted to each other as they assembled into graphene nanoribbons. With this method, the researchers were able to control the length, edge configuration, and location of the nanoribbons on the substrate.

This isn’t the first time that graphene nanoribbons were produced by self-assembly, but in earlier efforts the end results were bundles of ribbons that needed to go through another process to untangle them and position them in a device.

“Previous strategies in bottom-up molecular assemblies used inert substrates, such as gold or silver, to give molecules a lot of freedom to diffuse and react on the surface,” said Patrick Han of Tohoku University in the press release. “But this also means that the way these molecules assemble is completely determined by the intermolecular forces and by the molecular chemistry. Our method opens the possibility for self-assembling single-graphene devices at desired locations, because of the length and the direction control.”



IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.

Dexter Johnson
Madrid, Spain
Rachel Courtland
New York City
Load More